Lung growth beyond normal dimensions can be induced in adult rats in response to partial resection or physiological overload. Since all such methods involve heightened respiratory demands, experiments are planned in which the metabolic activities of animals will be altered in other ways. One approach will be to induce hypo- or hyperthyroidism by administration of propylthiouracil or triiodothyronine, respectively. Another will be to cause chronic anemia by repeated doses of phenylhydrazine. In all cases, various parameters of lung size will be analyzed at the end of the experiments to determine if growth or atrophy will have occurred. The effects of maternal pneumonectomy on fetal lung development will also be explored to learn if nonfunctional organs in prenatal animals can compensate for homologous deficiencies in their mothers. The outcome of such an investigation will reveal whether or not lung growth is regulated by mechanisms not related to respiratory demands. In order to study the effects of disuse on lung growth, combined heart and lung grafts will be transplanted to intraabdominal sites by microvascular anastomoses between the donor and host aortae and venae cavae in an inbred strain of rat.